Small pore molecular sieve supported copper catalysts durable against lean/rich aging for the reduction of nitrogen oxides

ABSTRACT

A method of using a catalyst comprises exposing a catalyst to at least one reactant in a chemical process. The catalyst comprises copper and a small pore molecular sieve having a maximum ring size of eight tetrahedral atoms. The chemical process undergoes at least one period of exposure to a reducing atmosphere. The catalyst has an initial activity and the catalyst has a final activity after the at least one period of exposure to the reducing atmosphere. The final activity is within 30% of the initial activity at a temperature between 200 and 500° C.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 13/334,259, filed Dec. 22, 2011 (pending), whichclaims priority to U.S. patent application Ser. No. 13/189,981, filedJul. 25, 2011 (granted as U.S. Pat. No. 8,101,147), which claimspriority to U.S. patent application Ser. No. 12/762,971, filed Apr. 19,2010 (granted as U.S. Pat. No. 7,998,443), which claims priority to U.S.Provisional Application No. 61/170,358, filed Apr. 17, 2009, and U.S.Provisional Application No. 61/312,832, filed Mar. 11, 2010, thedisclosures all of which are incorporated herein by reference in theirentireties for all purposes.

FIELD OF THE INVENTION

The present invention relates to small pore molecular sieve supportedcopper catalysts that are durable after being exposed to a reducingatmosphere, particularly after high temperature exposure.

BACKGROUND OF THE INVENTION

Selective catalytic reduction (SCR) of NO_(x) by nitrogenous compounds,such as ammonia or urea, has developed for numerous applicationsincluding for treating industrial stationary applications, thermal powerplants, gas turbines, coal-fired power plants, plant and refineryheaters and boilers in the chemical processing industry, furnaces, cokeovens, municipal waste plants and incinerators, and a number ofvehicular (mobile) applications, e.g., for treating diesel exhaust gas.

Several chemical reactions occur in an NH₃ SCR system, all of whichrepresent desirable reactions that reduce NO_(x) to nitrogen. Thedominant reaction is represented by reaction (1).

4NO+4NH₃+O₂→4N₂+6H₂O  (1)

Competing, non-selective reactions with oxygen can produce secondaryemissions or may unproductively consume ammonia. One such non-selectivereaction is the complete oxidation of ammonia, shown in reaction (2).

4NH₃+5O₂→4NO+6H₂O  (2)

Also, side reactions may lead to undesirable products such as N₂O, asrepresented by reaction (3).

4NH₃+4NO+3O₂→4N₂O+6H₂O  (3)

Catalysts for SCR of NO_(x) with NH₃ may include, for example,aluminosilicate molecular sieves. One application is to control NO_(x)emissions from vehicular diesel engines, with the reductant obtainablefrom an ammonia precursor such as urea or by injecting ammonia per se.To promote the catalytic activity, transition metals may be incorporatedinto the aluminosilicate molecular sieves. The most commonly testedtransition metal molecular sieves are Cu/ZSM-5, Cu/Beta, Fe/ZSM-5 andFe/Beta because they have a relatively wide temperature activity window.In general, however, Cu-based molecular sieve catalysts show better lowtemperature NO_(x) reduction activity than Fe-based molecular sievecatalysts.

In use, ZSM-5 and Beta molecular sieves have a number of drawbacks. Theyare is susceptible to dealumination during high temperature hydrothermalageing resulting in a loss of acidity, especially with Cu/Beta andCu/ZSM-5 catalysts. Both beta- and ZSM-5-based catalysts are alsoaffected by hydrocarbons which become adsorbed on the catalysts atrelatively low temperatures and are oxidized as the temperature of thecatalytic system is raised, generating a significant exotherm, which canthermally damage the catalyst. This problem is particularly acute invehicular diesel applications where significant quantities ofhydrocarbon can be adsorbed on the catalyst during cold-start; and Betaand ZSM-5 molecular sieves are also prone to coking by hydrocarbons.

In general, Cu-based molecular sieve catalysts are less thermallydurable, and produce higher levels of N₂O than Fe-based molecular sievecatalysts. However, they have a desirable advantage in that they slipless ammonia in use compared with a corresponding Fe-molecular sievecatalyst.

WO 2008/132452 discloses a method of converting nitrogen oxides in a gasto nitrogen by contacting the nitrogen oxides with a nitrogenousreducing agent in the presence of a zeolite catalyst containing at leastone transition metal, wherein the zeolite is a small pore zeolitecontaining a maximum ring size of eight tetrahedral atoms, wherein theat least one transition metal is selected from the group consisting ofCr, Mn, Fe, Co, Ce, Ni, Cu, Zn, Ga, Mo, Ru, Rh, Pd, Ag, In, Sn, Re, Irand Pt.

WO 2008/106518 discloses a combination of a fiber matrix wall flowfilter and a hydrophobic chabazite molecular sieve as a SCR catalyst onthe fiber matrix wall flow filter. The filter purportedly achievesimproved flexibility in system configuration and lower fuel costs foractive regeneration. Such active regeneration would likely encompassexposure to lean atmospheric conditions. The reference, however, doesnot contemplate subjecting the filter to reducing conditions. Thereference also fails to disclose or appreciate maintaining thedurability of a catalyst after being exposed to such a reducingatmosphere.

SUMMARY OF THE INVENTION

According to one embodiment of the present invention, a method of usinga catalyst comprises exposing a catalyst to at least one reactant in achemical process. The catalyst comprises copper and a small poremolecular sieve having a maximum ring size of eight tetrahedral atoms.Preferably, the catalyst is a copper promoted small pore molecularsieve, i.e., a small pore molecular sieve loaded with copper. Thechemical process undergoes at least one period of exposure to a reducingatmosphere. The catalyst has an initial activity and the catalyst has afinal activity after the at least one period of exposure to the reducingatmosphere. The final activity is within 30% of the initial activity ata temperature between 200 and 500° C.

According to another embodiment of the present invention, a method ofusing a catalyst comprises exposing a catalyst to at least one reactantcomprising nitrogen oxides in a chemical process comprising exhaust gastreatment. The catalyst comprises copper and a small pore molecularsieve having a maximum ring size of eight tetrahedral atoms selectedfrom the group of Framework Type Codes consisting of CHA, LEV, ERI andDDR. The chemical process undergoes at least one period of exposure to areducing atmosphere. The catalyst has an initial activity, and thecatalyst has a final activity after the at least one period of exposureto the reducing atmosphere. The final activity is within 10% of theinitial activity at a temperature between 250 and 350° C.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be more fully understood, reference ismade to the following drawing by way of illustration only, in which:

FIG. 1 is a graph illustrating NO_(x) conversion of medium pore andlarge pore molecular sieve supported copper catalysts after leanhydrothermal aging and lean/rich cycle aging;

FIG. 2 is a graph illustrating NO_(x) conversion of Fe/molecular sievecatalysts after lean hydrothermal aging and lean/rich cycle aging;

FIG. 3 is a graph illustrating NO_(x) conversion of small pore molecularsieve supported copper catalysts according to embodiments of theinvention and a comparative Cu/Beta catalyst after lean hydrothermalaging and lean/rich cycle aging; and

FIG. 4 is a graph illustrating NO_(x) conversion efficiency over a NACand combined NAC+SCR systems with different SCR catalysts according toembodiments of is the invention and comparative examples.

DETAILED DESCRIPTION OF THE INVENTION

A method of treating NO_(x) in an exhaust gas of a lean burn internalcombustion engine is to store the NO_(x) from a lean gas in a basicmaterial and then to release the NO_(x) from the basic material andreduce it periodically using a rich gas. The combination of a basicmaterial (such as an alkali metal, alkaline earth metal or a rare earthmetal), and a precious metal (such as platinum), and possibly also areduction catalyst component (such as rhodium) is typically referred toas a NO_(x) adsorber catalyst (NAC), a lean NO_(x) trap (LNT), or aNO_(x) storage/reduction catalyst (NSRC). As used herein, NO_(x)storage/reduction catalyst, NO_(x) trap, and NO_(x) adsorber catalyst(or their acronyms) may be used interchangeably.

Under certain conditions, during the periodically rich regenerationevents, NH₃ may be generated over a NO_(x) adsorber catalyst. Theaddition of a SCR catalyst downstream of the NO_(x) adsorber catalystmay improve the overall system NO_(x) reduction efficiency. In thecombined system, the SCR catalyst is capable of storing the released NH₃from the NAC catalyst during rich regeneration events and utilizes thestored NH₃ to selectively reduce some or all of the NO_(x) that slipsthrough the NAC catalyst during the normal lean operation conditions. Asused herein, such combined systems may be shown as a combination oftheir respective acronyms, e.g., NAC+SCR or LNT+SCR.

The combined NAC+SCR system imposes additional requirements on the SCRcatalyst component. Namely, besides having good activity and excellentthermal stability, the SCR catalyst has to be stable against lean/richexcursions. Such lean/rich excursions not only may occur during theregular NAC regeneration events, but also may happen during the NACdesulfation events. During the NAC desulfation events, the SCR catalystmay be exposed to temperatures much higher than it would be exposed toduring the regular NO_(x) regeneration events. Therefore, a good SCRcatalyst that is suitable for the NAC+SCR systems needs to be durableafter being exposed to a reducing atmosphere at high temperature.Although the present invention is described herein with particularemphasis on the SCR embodiment, it is contemplated that the presentinvention may encompass any catalysts which lose activity when exposedto a reducing atmosphere.

Catalysts are often unstable when exposed to a reducing atmosphere, moreparticularly a high temperature reducing atmosphere. For example, coppercatalysts are unstable during repeated lean/rich high temperatureexcursions, e.g., as is often encountered in vehicle exhaust gas or anexhaust gas treatment system. The reducing atmosphere occurs in the richphase of a lean/rich excursion cycle. The reducing atmosphereconditions, however, can occur in a variety of environments includingbut not limited to environments typical for the regeneration or thedesulfation of a NO_(x) adsorber catalyst, and for the activeregeneration of a catalyzed soot filter, etc. Thus, as used herein, areducing atmosphere is net reducing, for example, an exhaust gas havinga lambda value of less than 1 (e.g., derived from an air/fuel ratio lessthan stoichiometric). Contrastingly, a non-reducing atmosphere is netoxidizing, e.g., having a lambda value greater than 1 (e.g., derivedfrom an air/fuel ratio greater than stoichiometric).

Without wishing to be bound to a particular theory, it was believedprior to discovery of the present invention that molecular sievesupported copper catalysts would not maintain stability or activity whenexposed to a reducing atmosphere (especially a reducing atmosphereencountered in a repeated lean/rich cycle excursions) because whenexposed to the reducing atmosphere, the copper catalysts lost theiractivity. This loss of activity was suspected to be due to coppermigration, sintering, and/or reduced copper dispersion. Surprisingly, wediscovered in the present invention that small pore molecularsieve-supported copper catalysts maintained their catalytic activityeven though the medium and large pore molecular sieve supported coppercatalysts could not. It is believed that small pore molecular sievesprovide a restriction on the copper from migrating out of the framework,sintering, losing copper dispersion, and beneficially resulting in animproved stability and activity of the catalyst. The medium and largepore molecular sieves, however, do not maintain their stability andactivity when exposed to a reducing atmosphere possibly because of theeffects of copper migration, sintering, and/or reduced copperdispersion.

According to one embodiment of the present invention, a method of usinga catalyst comprises exposing a catalyst to at least one reactant in achemical process. The catalyst comprises copper and a small poremolecular sieve having a maximum ring size of eight tetrahedral atoms.The chemical process undergoes at least one period of exposure to areducing atmosphere. The catalyst has an initial activity and thecatalyst has a final activity after the at least one period of exposureto the reducing atmosphere. The final activity is within 30% of theinitial activity at a temperature between 150 and 650° C., preferablybetween 200 and 500° C.

A method of using a catalyst comprises exposing a catalyst to at leastone reactant in a chemical process. As used herein, chemical process caninclude any suitable chemical process using a catalyst comprising asmall pore molecular sieve comprising copper and encountering reducingconditions. Typical chemical processes include, but are not limited to,exhaust gas treatment such as selective catalytic reduction usingnitrogenous reductants, lean NO_(x) catalyst, catalyzed soot filter, ora combination of any one of these with a NO_(x) adsorber catalyst or athree-way catalyst (TWC), e.g., NAC+(downstream)SCR orTWC+(downstream)SCR.

According to another aspect of the invention, provided is a systemcomprising NAC+(downstream)SCR or TWC+(downstream)SCR, wherein the SCRcatalyst comprises a copper promoted small pore zeolite sieve asdescribed here.

According to another aspect of the invention, provided is an SCRcatalyzed soot filter wherein the SCR catalyst comprises a copperpromoted small pore zeolite sieve as described here.

A method of using a catalyst comprises exposing a catalyst to at leastone reactant. The reactant may include any reactants typicallyencountered in the chemical processes above. Reactants may include aselective catalytic reductant, such as ammonia. Selective catalyticreduction may include (1) using ammonia or a nitrogenous reductant or(2) a hydrocarbon reductant (the latter also known as lean NO_(x)catalysis). Other reactants may include nitrogen oxides and oxygen.

The catalyst comprises a transition metal, preferably copper, and asmall pore molecular sieve having a maximum ring size of eighttetrahedral atoms. As is used is herein “molecular sieve” is understoodto mean a metastable material containing tiny pores of a precise anduniform size that may be used as an adsorbent for gases or liquids. Themolecules which are small enough to pass through the pores are adsorbedwhile the larger molecules are not. The molecular sieve framework may bedefined as is generally acceptable by the International ZeoliteAssociation framework type codes (at http://www.iza-online.org/). Thesemolecular sieves are described in more detail below.

Molecular sieves are typically defined by the member rings as follows:large pore rings are 12-member rings or larger; medium pore rings are10-member rings; and small pore rings are 8-member rings or smaller. Thecatalyst in the present invention is a small pore ring having a maximumring size of eight tetrahedral atoms.

Most catalysts are supported on medium pore (10-ring, such as ZSM-5) orlarge pore (12-ring, such as Beta) molecular sieves. A molecular sievesupported copper SCR catalyst, for example, may exhibit wide temperaturewindows under NO only conditions. These catalysts, however, are notstable against repeated lean/rich high temperature aging as isdemonstrated in FIG. 1. In FIG. 1, a Cu/Beta catalyst (large pore) and aCu/ZSM-5 catalyst (medium pore) are shown under hydrothermal agingconditions and lean/rich aging conditions. As is evidenced by the dottedlines representing the lean/rich aging conditions, these types ofcatalysts are not suitable when exposed to repeated reducing conditions.In particular, these catalysts are not suitable for NAC+SCRapplications.

Molecular sieve supported iron SCR catalysts, although not as active asmolecular sieve supported copper catalysts at low temperatures (e.g.<350° C.), are stable against repeated lean/rich high temperature agingas shown in FIG. 2. In FIG. 2, Fe/Ferrierite, Fe/ZSM-5, and Fe/Beta areshown after hydrothermal aging and lean/rich aging conditions.Accordingly, molecular sieve supported iron catalysts have been thetechnology of choice due to their excellent stability against cycledlean/rich aging, e.g., as is encountered in NAC+SCR applications.

Small pore molecular sieve supported Cu catalysts have been demonstratedto exhibit improved NH₃—SCR activity and excellent thermal stability.According to one aspect of the invention, it was found that this type ofcatalyst also tolerates repeated is lean/rich high temperature aging.FIG. 3 compares a series of small pore molecular sieve supported Cucatalysts (Cu/SAPO-34, Cu/Nu-3, and Cu/SSZ-13, respectively) against acomparative large pore catalyst (Cu/Beta) after 700° C./2 hourshydrothermal aging and 600° C./12 hours cycled lean/rich aging,respectively. As is evident in FIG. 3, the catalysts with small poremolecular sieve are very stable against lean/rich aging. In particular,the Cu/SAPO-34 catalyst exhibited exceptionally good low temperatureactivity and showed no activity degradation after cycled lean/richaging, i.e., repeated exposure to a reducing atmosphere.

The catalysts in embodiments of the present invention show a much widertemperature window of high NO_(x) conversion. The temperature range ofimproved conversion efficiency may range from about 150 to 650° C., moreparticularly from 200 to 500° C., more particularly from 200 to 450° C.,or most significantly from about 200 to 400° C. In these temperatureranges, conversion efficiencies after exposure to a reducing atmosphere,and even after exposure to a reducing atmosphere and to hightemperatures (e.g., up to 850° C.), may range from greater than 55% to100%, more preferably greater than 90% efficiency, and even morepreferably greater than 95% efficiency. In particular, combined NAC+SCRsystems show a much wider temperature window of high NO_(x) conversioncompared to either NAC catalysts alone or NAC+SCR systems using aFe/molecular sieve SCR catalyst. See FIG. 4. For example at about 250°C. and about 300° C., the NO_(x) conversion efficiencies for systemssubjected to lean/rich aging are as follows:

System (undergone lean/rich NO_(X) Conversion NO_(X) Conversion aging) %at 250° C. % at 300° C. NAC alone 73 92 NAC + Fe/Beta SCR catalyst 87 90NAC + Cu/SSZ-13 SCR catalyst 93 97 NAC + Cu/SAPO-34 SCR catalyst 97 96

As is evident from these results, the use of the NAC+Cu/small poremolecular sieve catalyst shows dramatic improvement in conversionefficiencies. These improvements are to the final NO_(x) emissions.Thus, an improvement from about 87% NO_(x) conversion (about 13% NO_(x)remaining) to about 97% NO_(x) conversion (about 3% NO_(x) remaining) isabout a 433% improvement in efficiency, based on the percent NO_(x)remaining.

The catalyst has an initial activity and the catalyst has a finalactivity after the at least one period of exposure to the reducingatmosphere. In certain embodiments, catalyst activity is NOx conversionefficiency. Accordingly, the initial activity is NOx conversionefficiency of a catalyst that has not been exposed to a reducingatmosphere and the final activity is NOx conversion efficiency of thecatalyst after exposure to a is reducing atmosphere. The initialactivity may include a baseline aging under hydrothermal conditions.Hydrothermal conditions may include aging at 700° C. for 2 hours with 5%H₂O in air.

The chemical process undergoes at least one period of exposure to areducing atmosphere. The reducing atmosphere may include any suitablereducing atmosphere such as during rich conditions in a lean/rich agingcycle. For example, a localized reducing atmosphere may also occurduring catalyzed soot filter regeneration. The at least one period ofexposure may include repeated exposures to reducing conditions or aprolonged exposure to reducing conditions. For example, a repeatedexposure may include a cycled lean/rich aging at 600° C. for 12 hours. Alean cycle may last from 15 seconds to several tens of minutes, and arich cycle may last from less than 1 second to several minutes. In aNAC-SCR system or TWC-SCR system, the rich cycle may range, for example,from 1 to 60 continuous seconds, from 1 to 15 continuous seconds, orfrom 5 to 15 continuous seconds. In a coated soot filter application(e.g., a SCR/DPF (diesel particulate filter), the rich cycle may range,for example, from 30 seconds to 60 minutes of continuous exposure, from5 minutes to 30 minutes of continuous exposure, or from 10 minutes to 30minutes of continuous exposure. For example, the lean portion of thecycle may consist of exposure to 200 ppm NO, 10% O₂, 5% H₂O, 5% CO₂ inN₂, and the rich portion of the cycle may consist of exposure to 200 ppmNO, 5000 ppm C₃H₆, 1.3% H₂, 4% CO, 1% O₂, 5% H₂O, 5% CO₂ in N₂. Thereducing atmosphere may be a high temperature reducing atmosphere. Ahigh temperature reducing atmosphere may occur at a temperature fromabout 150° C. to 850° C. or more particularly from about 450° C. to 850°C.

The final activity is within about 30%, more preferably within about10%, more preferably within about 5%, even more preferably within about3% of the initial activity at a catalytic operating temperature.Preferably, the catalytic operating temperature is between about 150 andabout 650° C. and more preferably between about 200 and about 500° C.While the activity of the catalyst is preferably measured within atemperature range of 200 and 500° C., portions of the chemical processcan operate at any temperature, for example, a wider temperature rangeincluding higher temperatures. For example, catalyst activity will stillbe maintained in the temperature range of 200 and 500° C. even after thecatalyst has been exposed to higher temperatures, e.g. up to 850° C. Asused herein, when the final activity is given as a percentage of theinitial activity, it is given as an average of percentages over thetemperature range provided; in other words, if a final activity is saidto be within 30% of the initial activity at a temperature between 200and 500° C., it need not be less than 30% at every temperatures testedin that range, but would merely average less than 30% over thetemperatures tested. Moreover, while activity is identified as NO_(x)conversion in the examples of this application, the activity could besome other measure of catalyst activity depending on the chemicalprocess, as is known in the art. The data showing catalyst activity andthe percentage of initial activity to final activity is evidenced in thefollowing tables (See also FIG. 3). A negative number means that theactivity after exposure to the reducing conditions actually improvedrelative to the initial activity (and therefore would certainly be“within” a certain positive percentage of the initial activity):

For the embodiment using Cu/Nu-3 the following data was obtained:

TEMP HT Aging TEMP LR Aging % 150 9 150 9 −2% 200 50 198 52 −2% 250 76250 75 1% 350 72 350 69 4% 450 62 450 58 6% 550 45 550 43 3% 650 27 65026 2%

Thus, the lean/rich aging % NO_(x) reduction was within about 6% of thehydrothermal aging % NO_(x) reduction. Accordingly, the catalystremained stable and had good activity following repeated exposure toreducing conditions throughout the temperature ranging from about 150 toabout 650° C.

For the embodiment using Cu/SSZ-13 the following data was obtained:

TEMP HT Aging TEMP LR Aging % 164 61 160 19 68% 218 100 216 71 29% 269100 269 97 3% 373 97 372 86 12% 473 86 474 68 20% 572 64 573 41 36% 66822 669 −7 134%

Thus, the lean/rich aging % NO_(x) reduction was within about 30% of thehydrothermal aging % NO_(x) reduction throughout the temperature rangingfrom about 200 to about 500° C.

For the embodiment using Cu/SAPO34 the following data was obtained:

TEMP HT Aging TEMP LR Aging % 156 33 156 47 −41% 211 95 212 97 −3% 26499 265 99 0.04% 366 89 366 89 −0.01% 464 86 465 83 3% 561 70 564 62 11%658 34 662 26 22%

Thus, the lean/rich aging % NO_(x) reduction was within about 3% of thehydrothermal aging % NO_(x) reduction throughout the temperature rangingfrom about 200 to about 500° C., and within about 10% at temperaturesranging from about 200 to 560° C.

As a comparative example a Cu/Beta, large pore molecular sieve catalyst,was compared:

TEMP HT Aging TEMP LR Aging % 150 21 152 9 57% 200 72 199 24 67% 250 93250 30 67% 350 93 351 26 72% 450 82 450 37 56% 550 82 550 54 35% 650 64650 49 24%

The Cu/Beta comparative example showed poor activity following lean/richcycled aging. Thus, exposure to a reducing atmosphere for the coppermolecular sieve catalysts causes poor stability and activity as wascontemplated prior to discovery of the present invention.

According to one embodiment of the present invention, a method of usinga catalyst comprises exposing a catalyst to at least one reactantcomprising nitrogen oxides in a chemical process comprising exhaust gastreatment. The catalyst comprises copper and a small pore molecularsieve having a maximum ring size of eight tetrahedral atoms selectedfrom the group of Framework Type Codes consisting of CHA, LEV, ERI andDDR. The chemical process undergoes at least one period of exposure to areducing atmosphere. The catalyst has an initial activity, and thecatalyst has a final activity after the at least one period of exposureto the reducing atmosphere. The final activity is within 10% of theinitial activity at a temperature between 250 and 350° C. In a preferredembodiment, the catalyst has a final activity that is within 3% of theinitial activity at a temperature between 250 and 350° C.

In an embodiment of the invention, the catalysts have been combined witha NAC (NO_(x) adsorber catalyst) and tested as NAC+SCR systems. FIG. 4compares the NO_(x) reduction efficiency over a NAC alone and NAC+SCRsystems with different SCR small pore molecular sieve catalysts(Cu/SAPO-34, and Cu/SSZ-13), and a comparative example of a Fe/betacatalyst. Combining an Fe/molecular sieve SCR with a NAC catalyst hasbeen shown to improve the system NO_(x) conversion compared to NACalone. Remarkably, however, the other two systems with a small poremolecular sieve comprising copper, i.e., Cu/SAPO-34 or Cu/SSZ-13, alsoexhibited further improved NO_(x) removal efficiency. This is especiallyevident at low temperatures (200-350° C.). These results clearly suggestthat small pore molecular sieve supported Cu catalysts offers newpotential to further improve the performance of NAC+SCR systems.

In addition to NAC+SCR applications, the small pore molecular sievesupported Cu catalysts offer significant performance advantages forother applications that may be exposed to a high temperature reducingatmosphere. For example, a small pore molecular sieve supported Cucatalyst may be used in a reducing atmosphere which occurs during activeregeneration of a SCR/DPF (diesel particulate filter). Small poremolecular sieve supported Cu catalysts provide excellent thermaldurability and exceptional stability against reducing conditions, e.g.,rich aging that occurs in exhaust is gas treatment systems.

It will be appreciated that by defining the molecular sieve by theirFramework Type Codes we intend to include the “Type Material” and anyand all isotypic framework materials. (The “Type Material” is thespecies first used to establish the framework type). Reference is madeto Table 1, which lists a range of illustrative molecular sievematerials for use in the present invention. For the avoidance of doubt,unless otherwise made clear, reference herein to a molecular sieve byname, e.g. “chabazite”, is to the molecular sieve material per se (inthis example the naturally occurring type material chabazite) and not toany other material designated by the Framework Type Code to which theindividual molecular sieve may belong, e.g. some other isotypicframework material. Use of a FTC herein is intended to refer to the TypeMaterial and all isotypic framework materials defined by that FTC.

The distinction between molecular sieve type materials, such asnaturally occurring (i.e. mineral) chabazite, and isotypes within thesame Framework Type Code is not merely arbitrary, but reflectsdifferences in the properties between the materials, which may in turnlead to differences in activity in the method of the present invention.It will be appreciated, e.g. from Table 1 hereinbelow, that by “MeAPSO”and “MeAlPO” we intend zeotypes substituted with one or more metals.Suitable substituent metals include one or more of, without limitation,As, B, Be, Co, Fe, Ga, Ge, Li, Mg, Mn, Zn and Zr.

In a particular embodiment, the small pore molecular sieve catalysts foruse in the present invention can be selected from the group consistingof aluminosilicate molecular sieves, metal-substituted aluminosilicatemolecular sieves and aluminophosphate molecular sieves. Aluminophosphatemolecular sieves with application in the present invention includealuminophosphate (AlPO) molecular sieves, metal substituted (MeAlPO)molecular sieves, silico-aluminophosphate (SAPO) molecular sieves andmetal substituted silico-aluminophosphate (MeAPSO) molecular sieves.

In one embodiment, the small pore molecular sieve is selected from thegroup of Framework Type Codes consisting of: ACO, AEI, AEN, AFN, AFT,AFX, ANA, APC, APD, ATT, CDO, CHA, DDR, DFT, EAB, EDI, EPI, ERI, GIS,GOO, IHW, ITE, ITW, LEV, KFI, MER, MON, NSI, OWE, PAU, PHI, RHO, RTH,SAT, SAV, SIV, THO, TSC, UEI, UFI, VNI, YUG and ZON.

In an embodiment, the small pore molecular sieve containing a maximumring size of eight tetrahedral atoms is selected from the group ofFramework Type Codes consisting of CHA, LEV, ERI, and DDR. In apreferred embodiment, the small pore molecular sieve comprises a CHAFramework Type Code selected from SAPO-34 or SSZ-13. In anotherembodiment, the small pore molecular sieve comprises a LEV FrameworkType Code Nu-3. Additionally, the small pore molecular sieve maycomprise an AEI Framework Type Code SAPO-18, an ERI Framework Type CodeZSM-34, and/or a DDR Framework Type Code sigma-1. The small poremolecular sieve may also include disordered molecular sieves, such as anintergrown or mixed phase AEI/CHA, AEI/SAV, etc.

Molecular sieves with application in the present invention can includethose that have been treated to improve hydrothermal stability.Illustrative methods of improving hydrothermal stability include:

(i) Dealumination by: steaming and acid extraction using an acid orcomplexing agent e.g. (EDTA—ethylenediaminetetracetic acid); treatmentwith acid and/or complexing agent; treatment with a gaseous stream ofSiCl₄ (replaces Al in the molecular sieve framework with Si);

(ii) Cation exchange—use of multi-valent cations such as La; and

(iii) Use of phosphorous containing compounds (see e.g. U.S. Pat. No.5,958,818).

Illustrative examples of suitable small pore molecular sieves are setout in Table 1.

TABLE 1 Small Pore Molecular Sieve Molecular Sieve Framework Typematerial* and Type (by illustrative isotypic Framework frameworkDimension- Additional Type Code) structures ality Pore size (Å) info ACO*ACP-1 3D 3.5 × 2.8, 3.5 × Ring sizes—8, 4 3.5 AEI *AlPO-18 3D 3.8 × 3.8Ring sizes—8, 6, 4 [Co—Al—P—O]-AEI SAPO-18 SIZ-8 SSZ-39 AEN *AlPO-EN3 2D4.3 × 3.1, 2.7 × Ring sizes—8, 6, 4 5.0 AlPO-53(A) AlPO-53(B)[Ga—P—O]-AEN CFSAPO-1A CoIST-2 IST-2 JDF-2 MCS-1 MnAPO-14 Mu-10UiO-12-500 UiO-12-as AFN *AlPO-14 3D 1.9 × 4.6, 2.1 × Ring sizes—8, 6, 44.9, 3.3 × 4.0 |(C₃N₂H₁₂)—|[Mn—Al—P—O]- AFN GaPO-14 AFT *AlPO-52 3D 3.8× 3.2, 3.8 × Ring sizes—8, 6, 4 3.6 AFX *SAPO-56 3D 3.4 × 3.6 Ringsizes—8, 6, 4 MAPSO-56, M = Co, Mn, Zr SSZ-16 ANA *Analcime 3D 4.2 × 1.6Ring sizes—8, 6, 4 AlPO₄-pollucite AlPO-24 Ammonioleucite[Al—Co—P—O]-ANA [Al—Si—P—O]-ANA |Cs—|[Al—Ge—O]-ANA |Cs—|[Be—Si—O]-ANA|Cs₁₆|[Cu₈Si₄₀O₉₆]- ANA |Cs—Fe|[Si—O]-ANA |Cs—Na—(H₂O)|[Ga—Si—O]- ANA[Ga—Ge—O]-ANA |K—|[B—Si—O]-ANA |K—|[Be—B—P—O]-ANA |Li—|[Li—Zn—Si—O]-ANA|Li—Na|[Al—Si—O]-ANA |Na—|[Be—B—P—O]- ANA |(NH₄)—|[Be—B—P—O]- ANA|(NH₄)—|[Zn—Ga—P—O]- ANA [Zn—As—O]-ANA Ca-D Hsianghualite Leucite Na—BPollucite Wairakite APC *AlPO—C 2D 3.7 × 3.4, 4.7 × Ring sizes—8, 6, 42.0 AlPO—H3 CoAPO-H3 APD *AlPO-D 2D 6.0 × 2.3, 5.8 × Ring sizes—8, 6, 41.3 APO-CJ3 ATT *AlPO-12-TAMU 2D 4.6 × 4.2, 3.8 × Ring sizes—8, 6, 4 3.8AlPO-33 RMA-3 CDO *CDS-1 2D 4.7 × 3.1, 4.2 × Ring sizes—8, 5 2.5 MCM-65UZM-25 CHA *Chabazite 3D 3.8 × 3.8 Ring sizes—8, 6, 4 AlPO-34[Al—As—O]-CHA [Al—Co—P—O]-CHA |Co| [Be—P—O]-CHA |Co₃ (C₆N₄H₂₄)₃ (H₂O)₉|[Be₁₈P₁₈O₇₂]- CHA [Co—Al—P—O]-CHA |Li—Na| [Al—Si—O]- CHA [Mg—Al—P—O]-CHA[Si—O]-CHA [Zn—Al—P—O]-CHA [Zn—As—O]-CHA CoAPO-44 CoAPO-47 DAF-5 GaPO-34K-Chabazite Linde D Linde R LZ-218 MeAPO-47 MeAPSO-47 (Ni(deta)₂)-UT-6Phi SAPO-34 SAPO-47 SSZ-13 UiO-21 Willhendersonite ZK-14 ZYT-6 DDR*Deca-dodecasil 3R 2D 4.4 × 3.6 Ring sizes—8, 6, 5, 4 [B—Si—O]-DDRSigma-1 ZSM-58 DFT *DAF-2 3D 4.1 × 4.1, 4.7 × Ring sizes—8, 6, 4 1.8ACP-3, [Co—Al—P—O]- DFT [Fe—Zn—P—O]-DFT [Zn—Co—P—O]-DFT UCSB-3GaGeUCSB-3ZnAs UiO-20, [Mg—P—O]- DFT EAB *TMA-E 2D 5.1 × 3.7 Ring sizes—8,6, 4 Bellbergite EDI *Edingtonite 3D 2.8 × 3.8, 3.1 × Ring sizes—8, 42.0 |(C₃H₁₂N₂)_(2.5)| [Zn₅P₅O₂₀]-EDI [Co—Al—P—O]-EDI [Co—Ga—P—O]-EDI|Li—|[Al—Si—O]-EDI |Rb₇ Na (H₂O)₃| [Ga₈Si₁₂O₄₀]-EDI [Zn—As—O]-EDI K—FLinde F Zeolite N EPI *Epistilbite 2D 4.5 × 3.7, 3.6 × Ring sizes—8, 43.6 ERI *Erionite 3D 3.6 × 5.1 Ring sizes—8, 6, 4 AlPO-17 Linde T LZ-220SAPO-17 ZSM-34 GIS *Gismondine 3D 4.5 × 3.1, 4.8 × Ring sizes—8, 4 2.8Amicite [Al—Co—P—O]-GIS [Al—Ge—O]-GIS [Al—P—O]-GIS [Be—P—O]-GIS|(C₃H₁₂N₂)₄| [Be₈P₈O₃₂]-GIS |(C₃H₁₂N₂)₄| [Zn₈P₈O₃₂]-GIS [Co—Al—P—O]-GIS[Co—Ga—P—O]-GIS [Co—P—O]-GIS |Cs₄|[Zn₄B₄P₈O₃₂]- GIS [Ga—Si—O]-GIS[Mg—Al—P—O]-GIS |(NH₄)₄|[Zn₄B₄P₈O₃₂]- GIS |Rb₄|[Zn₄B₄P₈O₃₂]- GIS[Zn—Al—As—O]-GIS [Zn—Co—B—P—O]-GIS [Zn—Ga—As—O]-GIS [Zn—Ga—P—O]-GISGarronite Gobbinsite MAPO-43 MAPSO-43 Na-P1 Na-P2 SAPO-43 TMA-gismondineGOO *Goosecreekite 3D 2.8 × 4.0, 2.7 × Ring sizes—8, 6, 4 4.1, 4.7 × 2.9IHW *ITQ-32 2D 3.5 × 4.3 Ring sizes—8, 6, 5, 4 ITE *ITQ-3 2D 4.3 × 3.8,2.7 × Ring sizes—8, 6, 5, 4 5.8 Mu-14 SSZ-36 ITW *ITQ-12 2D 5.4 × 2.4,3.9 × Ring sizes—8, 6, 5, 4 4.2 LEV *Levyne 2D 3.6 × 4.8 Ring sizes—8,6, 4 AlPO-35 CoDAF-4 LZ-132 NU-3 RUB-1 [B—Si—O]-LEV SAPO-35 ZK-20ZnAPO-35 KFI ZK-5 3D 3.9 × 3.9 Ring sizes—8, 6, 4 |18-crown-6|[Al—Si—O]-KFI [Zn—Ga—As—O]-KFI (Cs,K)-ZK-5 P Q MER *Merlinoite 3D 3.5 × 3.1, 3.6 ×Ring sizes—8, 4 2.7, 5.1 × 3.4, 3.3 × 3.3 [Al—Co—P—O]-MER|Ba—|[Al—Si—O]-MER |Ba—Cl—|[Al—Si—O]- MER [Ga—Al—Si—O]-MER|K—|[Al—Si—O]-MER |NH₄—|[Be—P—O]-MER K-M Linde W Zeolite W MON*Montesommaite 2D 4.4 × 3.2, 3.6 × Ring sizes—8, 5, 4 3.6 [Al—Ge—O]-MONNSI *Nu-6(2) 2D 2.6 × 4.5, 2.4 × Ring sizes—8, 6, 5 4.8 EU-20 OWE*UiO-28 2D 4.0 × 3.5, 4.8 × Ring sizes—8, 6, 4 3.2 ACP-2 PAU *Paulingite3D 3.6 × 3.6 Ring sizes—8, 6, 4 [Ga—Si—O]-PAU ECR-18 PHI *Phillipsite 3D3.8 × 3.8, 3.0 × Ring sizes—8, 4 4.3, 3.3 × 3.2 [Al—Co—P—O]-PHI DAF-8Harmotome Wellsite ZK-19 RHO *Rho 3D 3.6 × 3.6 Ring sizes—8, 6, 4[Be—As—O]-RHO [Be—P—O]-RHO [Co—Al—P—O]-RHO |H—|[Al—Si—O]-RHO[Mg—Al—P—O]-RHO [Mn—Al—P—O]-RHO |Na₁₆ Cs₈| [Al₂₄Ge₂₄O₉₆]-RHO|NH₄—|[Al—Si—O]-RHO |Rb—|[Be—As—O]-RHO Gallosilicate ECR-10 LZ-214Pahasapaite RTH *RUB-13 2D 4.1 × 3.8, 5.6 × Ring sizes—8, 6, 5, 4 2.5SSZ-36 SSZ-50 SAT *STA-2 3D 5.5 × 3.0 Ring sizes—8, 6, 4 SAV *Mg-STA-73D 3.8 × 3.8, 3.9 × Ring sizes—8, 6, 4 3.9 Co-STA-7 Zn-STA-7 SBN *UCSB-93D TBC Ring sizes—8, 4, 3 SU-46 SIV *SIZ-7 3D 3.5 × 3.9, 3.7 × Ringsizes—8, 4 3.8, 3.8 × 3.9 THO *Thomsonite 3D 2.3 × 3.9, 4.0 × Ringsizes—8, 4 2.2, 3.0 × 2.2 [Al—Co—P—O]-THO [Ga—Co—P—O]-THO|Rb₂₀|[Ga₂₀Ge₂₀O₈₀]- THO [Zn—Al—As—O]-THO [Zn—P—O]-THO [Ga—Si—O]-THO)[Zn—Co—P—O]-THO TSC *Tschörtnerite 3D 4.2 × 4.2, 5.6 × Ring sizes— 8, 6,4 3.1 UEI *Mu-18 2D 3.5 × 4.6, 3.6 × Ring sizes—8, 6, 4 2.5 UFI *UZM-52D 3.6 × 4.4, 3.2 × Ring sizes—8, 6, 4 3.2 (cage) VNI *VPI-9 3D 3.5 ×3.6, 3.1 × Ring sizes—8, 5, 4, 3 4.0 YUG *Yugawaralite 2D 2.8 × 3.6, 3.1× Ring sizes—8, 5, 4 5.0 Sr-Q ZON *ZAPO-M1 2D 2.5 × 5.1, 3.7 × Ringsizes—8, 6, 4 4.4 GaPO-DAB-2 UiO-7

Small pore molecular sieves with particular application for exposure toreducing conditions are set out in Table 2.

TABLE 2 Preferred Small Pore Molecular Sieves. Structure Molecular SieveCHA SAPO-34 AlPO-34 SSZ-13 LEV Levynite Nu-3 LZ-132 SAPO-35 ZK-20 ERIErionite ZSM-34 Linde type T DDR Deca-dodecasil 3R Sigma-1 KFI ZK-518-crown-6 [Zn—Ga—As—O]-KFI EAB TMA-E PAU ECR-18 MER Merlinoite AEISSZ-39 GOO Goosecreekite YUG Yugawaralite GIS P1 VNI VPI-9 AEI SAPO-18

Molecular sieves for use in the present application include natural andsynthetic molecular sieves, preferably synthetic molecular sievesbecause the molecular sieves can have a more uniform: silica-to-aluminaratio (SAR), crystallite size, crystallite morphology, and the absenceof impurities (e.g. alkaline earth metals). Small pore aluminosilicatemolecular sieves may have a silica-to-alumina ratio (SAR) of from 2 to300, optionally 4 to 200, and preferably 8 to 150. It will beappreciated that higher SAR ratios are preferred to improve thermalstability but this may negatively affect transition metal exchange.

The at least one reactant may contact the catalyst at a gas hourly spacevelocity of from 5,000 hr⁻¹ to 500,000 hr⁻¹, optionally from 10,000 hr⁻¹to 200,000 hr⁻¹.

Small pore molecular sieves for use in the invention may havethree-dimensional dimensionality, i.e. a pore structure which isinterconnected in all three crystallographic dimensions, ortwo-dimensional dimensionality. In one embodiment, the small poremolecular sieves for use in the present invention consist of molecularsieves having three-dimensional dimensionality. In another embodiment,the small pore molecular sieves for use in the present invention consistof molecular sieves having two-dimensional dimensionality.

In certain embodiments, the small pore molecular sieve comprises,consists essentially of, or consists of a disordered framework selectedfrom the group consisting of ABC-6, AEI/CHA, AEI/SAV, AEN/UEI, AFS/BPH,BEC/ISV, beta, fuajasite, ITE/RTH, KFI/SAV, lovdarite, montesommaite,MTT/TON, pentasils, SBS/SBT, SSF/STF, SSZ-33, and ZSM-48. In a preferredembodiment, one or more of the small pore molecular sieves may comprisea CHA Framework Type Code selected from SAPO-34, AlPO-34, SAPO-47,ZYT-6, CAL-1, SAPO-40, SSZ-62 or SSZ-13 and/or an AEI Framework TypeCode of selected from AlPO-18, SAPO-18, SIZ-8, or SSZ-39. In oneembodiment, the mixed phase composition is an AEI/CHA-mixed phasecomposition. The ratio of each framework type in the molecular sieve isnot particularly limited. For example, the ratio of AEI/CHA may rangefrom about 5/95 to about 95/5, preferably about 60/40 to 40/60. In anexemplary embodiment, the ratio of AEI/CHA may range from about 5/95 toabout 40/60. It is envisioned that a disordered molecular sieve, such asAEI/CHA, will be used as the support for one or more transition metals,such as Cr, Mn, Fe, Co, Ce, Ni, Cu, Zn, Ga, Mo, Ru, Rh, Pd, Ag, In, Sn,Re, Ir, Pt, and mixtures thereof, preferably Cr, Mn, Fe, Co, Ce, Ni, Cu,Rh, Pd, Pt, and mixtures thereof, more preferably Fe and/or Cu, and mostpreferably copper.

The total of the copper metal that can be included in the molecularsieve can be from 0.01 to 20 wt %, based on the total weight of thecatalyst. In one embodiment, the total of the copper that can beincluded can be from 0.1 to 10 wt %. In a particular embodiment, thetotal of copper that can be included is from 0.5 to 5 wt %. The coppermay be included in the molecular sieve by any feasible method. Forexample, it can be added after the molecular sieve has been synthesized,e.g. by incipient wetness or exchange process; or can be added duringmolecular sieve synthesis.

A preferred two dimensional small pore molecular sieve for use in thepresent invention consists of Cu/LEV, such as Cu/Nu-3, whereas apreferred copper-containing three dimensional small pore molecularsieve/aluminophosphate molecular sieve for use in the present inventionconsists of Cu/CHA, such as Cu/SAPO-34 or Cu/SSZ-13.

The molecular sieve catalysts for use in the present invention can becoated on a suitable substrate monolith or can be formed asextruded-type catalysts, but are preferably used in a catalyst coating.In one embodiment, the molecular sieve catalyst is coated on aflow-through monolith substrate (i.e. a honeycomb monolithic catalystsupport structure with many small, parallel channels running axiallythrough the entire part) or filter monolith substrate such as awall-flow filter etc. The molecular sieve catalyst for use in thepresent invention can be coated, e.g. as a washcoat component, on asuitable monolith substrate, such as a metal or ceramic flow throughmonolith substrate or a filtering substrate, such as a wall-flow filteror sintered metal or partial filter (such as is disclosed in WO 01/80978or EP 1057519, the latter document describing a substrate comprisingconvoluted flow paths that at least slows the passage of soottherethrough). Alternatively, the molecular sieves for use in thepresent invention can be synthesized directly onto the substrate.Alternatively, the molecular sieve catalysts according to the inventioncan be formed into an extruded-type flow through catalyst.

Washcoat compositions containing the molecular sieves for use in thepresent invention for coating onto the monolith substrate formanufacturing extruded type substrate monoliths can comprise a binderselected from the group consisting of alumina, silica, (non molecularsieve) silica-alumina, naturally occurring clays, TiO₂, ZrO₂, and SnO₂.

In one embodiment, the at least one reactant, e.g., nitrogen oxides, arereduced with the reducing agent at a temperature of at least 100° C. Inanother embodiment, the at least one reactant are reduced with thereducing agent at a temperature from about 150° C. to 750° C. In aparticular embodiment, the temperature range is from 175 to 550° C., ormore particularly from 175 to 400° C.

For a reactant including nitrogen oxides, the reduction of nitrogenoxides may be carried out in the presence of oxygen or in the absence ofoxygen. The source of nitrogenous reductant can be ammonia per se,hydrazine or any suitable ammonia precursor (such as urea ((NH₂)₂CO)),ammonium carbonate, ammonium carbamate, ammonium hydrogen carbonate orammonium formate.

The method may be performed on a gas derived from a combustion process,such as from an internal combustion engine (whether mobile orstationary), a gas turbine and coal or oil fired power plants. Themethod may also be used to treat gas from industrial processes such asrefining, from refinery heaters and boilers, furnaces, the chemicalprocessing industry, coke ovens, municipal waste plants andincinerators, coffee roasting plants, etc.

In a particular embodiment, the method is used for treating exhaust gasfrom a vehicular internal combustion engine with a lean/rich cycle, suchas a diesel engine, a gasoline engine, or an engine powered by liquidpetroleum gas or natural gas.

For a reactant including nitrogen oxides, the nitrogenous reductant maybe metered into the flowing exhaust gas only when it is determined thatthe molecular sieves catalyst is capable of catalyzing NO_(x) reductionat or above a desired efficiency, such as at above 100° C., above 150°C. or above 175° C. The determination by the control means can beassisted by one or more suitable sensor inputs indicative of a conditionof the engine selected from the group consisting of: exhaust gastemperature, catalyst bed temperature, accelerator position, mass flowof exhaust gas in the system, manifold vacuum, ignition timing, enginespeed, lambda value of the exhaust gas, the quantity of fuel injected inthe engine, the position of the exhaust gas recirculation (EGR) valveand thereby the amount of EGR and boost pressure.

Metering may be controlled in response to the quantity of nitrogenoxides in the exhaust gas determined either directly (using a suitableNO_(x) sensor) or indirectly, such as using pre-correlated look-uptables or maps—stored in the control means—correlating any one or moreof the abovementioned inputs indicative of a condition of the enginewith predicted NO_(x) content of the exhaust gas.

The entire contents of any and all patents and references cited hereinare incorporated herein by reference.

EXAMPLES

Although the invention is illustrated and described herein withreference to specific embodiments, the invention is not intended to belimited to the details shown. Rather, various modifications may be madein the details within the scope and range of equivalents of the claimsand without departing from the invention.

1. Steady State SCR Evaluation

Steady-state selective catalytic reduction (SCR) activity tests wereconducted in a quartz reactor of 24 inches in length, and uniformlyheated by two tube furnace units of 12 inches length. Experiments wereperformed at a gas hourly space velocity of 30,000 hr⁻¹ utilizingcatalyst dimensions of 1 inch diameter×1 inch length. All gas linesdirectly connected to the reactor were maintained at 130° C. by heatingtape to prevent gas species adsorption on the walls of the gas line.Water vapor was provided by a water bomb, which was constantlymaintained at 70° C.

Prior to reaching the catalyst bed, the feed gas was heated and mixedupstream in the reactor via an inert thermal mass. The temperature ofthe gas stream was monitored at the catalyst inlet, center of thecatalyst bed, and at the outlet by k-type thermocouples. Reacted feedgas was analyzed by a FTIR downstream from the catalyst bed at asampling rate of 1.25 s⁻¹. The composition of the inlet feed gas couldbe determined by sampling from a bypass valve located upstream of thereactor.

Steady-state SCR experiments were initially performed on catalystsamples that had been hydrothermally (HT) aged at 700° C. for 2 hours inthe presence of air containing 4.5% H₂O. All steady state experimentswere conducted using a feed gas of NO and NH₃ containing 350 ppm of NO,with an ammonia-to-NO (ANR) ratio of 1 (i.e., 350 ppm of NH₃). Theremainder of the feed gas composition was as follows: 14% O₂, 4.6% H₂O,5% CO₂, balance N₂. The steady-state NO_(x) conversion was determined atcatalyst bed temperatures of 150° C., 200° C., 250° C., 350° C., 450°C., 550° C., and 650° C.

Catalysts were then aged under lean-rich cycling conditions at 600° C.for 12 h. The lean portion of the cycle consisted of exposure to 200 ppmNO, 10% O₂, 5% H₂O, 5% CO₂ in N₂ for 5 seconds at a space velocity of30,000 h⁻¹. The rich portion of the cycle consisted of exposure to 200ppm NO, 5000 ppm C₃H₆, 1.3% H₂, 4% CO, 1% O₂, 5% H₂O, 5% CO₂ in N₂ for15 seconds. After the aging, steady-state SCR experiments were performedas described above.

In FIG. 3, the NO_(x) conversion efficiency is shown for embodiments ofthe present invention and a comparative example. Cu/SAPO-34, Cu/Nu-3 andCu/SSZ-13, small pore molecular sieve catalysts according to embodimentsof the invention, are shown after having undergone the above describedhydrothermal aging treatment and lean/rich aging treatment,respectively. A comparative example showing Cu/Beta, a large poremolecular sieve catalyst, is also shown after both hydrothermal agingand lean/rich aging. As is evident, the small pore molecular sievecatalysts demonstrated enhanced NO_(x) conversion efficienciesespecially in the temperature window of 200 to 500° C.

2. NAC+SCR Experiments

NO_(x) adsorber catalyst (NAC) and SCR cores were initiallyhydrothermally aged at 750° C. for 16 hours in a 4.5% H₂O in air gasmixture. Samples were then mounted in the same reactor setup describedabove with the NAC catalyst mounted directly in front of the SCRcatalyst. The catalysts were aged at 600° C. for 12 hours under thelean-rich aging conditions described above (5 seconds lean/15 secondsrich).

The catalysts were then cooled to 450° C. under lean-rich cycling (60seconds lean/5 seconds rich, same gas compositions). At 450° C., 25lean-rich cycles were completed (60 seconds lean/5 seconds rich) withthe last five cycles used to determine an average cycle NO_(x)conversion for the catalyst. After the 25^(th) cycle, the catalyst washeld under the lean gas composition for 5 minutes. The catalyst was thencooled and evaluated at 400° C., 350° C., 300° C., 250° C., 200° C., and175° C. following the aforementioned cycle procedure.

In FIG. 4, the NO_(x) conversion efficiency is shown for embodiments ofthe present invention and two comparative examples. NAC+Cu/SAPO-34 andNAC+Cu/SSZ-13, small pore molecular sieve catalysts according toembodiments of the invention are shown after having undergone the abovedescribed lean/rich aging treatment. A comparative example showing NACalone and NAC+Fe/Beta, a large pore molecular sieve catalyst, is alsoshown after lean/rich aging. As is evident, the small pore molecularsieve catalysts demonstrate enhanced NO_(x) conversion efficienciescomparable to and/or better than NAC alone or NAC+Fe/Beta, especially inthe temperature window of 250 to 450° C.

While preferred embodiments of the invention have been shown anddescribed herein, it will be understood that such embodiments areprovided by way of example only. Numerous variations, changes andsubstitutions will occur to those skilled in the art without departingfrom the spirit of the invention. Accordingly, it is intended that theappended claims cover all such variations as fall within the spirit andscope of the invention.

1. A system for treating exhaust gas comprising NOx absorber catalystdisposed upstream of an SCR catalyst, wherein said SCR catalystcomprises (i) copper and small-pore molecular sieve, and (ii) molecularsieve supported iron catalyst.
 2. The system of claim 1, wherein saidsmall-pore molecular sieve has a CHA structure.
 3. The system of claim2, wherein said small-pore molecular sieve has a silica-to-alumina ratioof about 8 to about
 150. 4. The system of claim 2, wherein said smallpore molecular sieve is SSZ-13.
 5. The system of claim 2, wherein saidsmall pore molecular sieve is SAPO-34.
 6. The system of claim 1, whereinsaid small-pore molecular sieve has a LEV structure.
 7. The system ofclaim 1, wherein said small-pore molecular sieve has an ERI structure.8. The system of claim 1, wherein said small-pore molecular sieve has aDDR structure.
 9. The system of claim 1, wherein said molecular sievesupported iron catalyst is selected from the group consisting ofFe/Ferrierite, Fe/ZSM-5, and Fe/Beta.
 10. The system of claim 2, whereinsaid molecular sieve supported iron catalyst is selected from the groupconsisting of Fe/Ferrierite, Fe/ZSM-5, and Fe/Beta.
 11. The system ofclaim 3, wherein said molecular sieve supported iron catalyst isselected from the group consisting of Fe/Ferrierite, Fe/ZSM-5, andFe/Beta.
 12. The system of claim 4, wherein said molecular sievesupported iron catalyst is selected from the group consisting ofFe/Ferrierite, Fe/ZSM-5, and Fe/Beta.
 13. The system of claim 5, whereinsaid molecular sieve supported iron catalyst is selected from the groupconsisting of Fe/Ferrierite, Fe/ZSM-5, and Fe/Beta.
 14. The system ofclaim 6, wherein said molecular sieve supported iron catalyst isselected from the group consisting of Fe/Ferrierite, Fe/ZSM-5, andFe/Beta.
 15. The system of claim 7, wherein said molecular sievesupported iron catalyst is selected from the group consisting ofFe/Ferrierite, Fe/ZSM-5, and Fe/Beta.
 16. The system of claim 8, whereinsaid molecular sieve supported iron catalyst is selected from the groupconsisting of Fe/Ferrierite, Fe/ZSM-5, and Fe/Beta.